The present invention relates generally to air bags of the type utilized in vehicle occupant restraint systems. More particularly, the present invention relates to a vehicle air bag constructed substantially entirely of an uncoated fabric, as well as methods of producing such an air bag.
Virtually all motor vehicles in service today are equipped with seatbelts to restrain vehicle occupants during a collision. Recently, however, many vehicles have also been equipped with air bag systems to supplement the protection provided by seatbelts. These air bag systems utilize at least one folded air bag in fluid communication with a source of inflation gas. A sensor is provided to detect a collision between the vehicle and another object. When such a collision is detected, the sensor actuates the source of inflation gas. As a result, the air bag is rapidly expanded to absorb at least a portion of the impact force which would otherwise have been imparted to the vehicle occupant.
Typically, the air bag is designed to inflate in a period which generally corresponds to the "crash pulse" of the vehicle in which it is installed. For example, a vehicle having relatively "stiff" frame may have a crash pulse of approximately 30 milliseconds. In other words, a period of 30 milliseconds will elapse between the time in which a collision occurs at the front end of the vehicle and the time in which the force of such a collision is transmitted back to the vehicle occupant and the cushion is fully inflated.
In contrast, a vehicle having a relatively "soft" frame may have a crash pulse of approximately 50 milliseconds or more. Thus, an air bag installed in an exemplary vehicle having a relatively "stiff" frame may be required to inflate 20 milliseconds or more faster than an air bag installed in an exemplary vehicle have a relatively "soft" frame. To effect this faster inflation, a larger and more powerful source of inflation gas will typically be required.
Air bags have generally been divided into two types, i.e. driver side and passenger side. Driver side air bags have often been fitted into the vehicle steering column. These air bags, which typically have a circular configuration when fully inflated, have tended to be smaller because of the relatively small space between the driver and the steering wheel.
Passenger side air bags, on the other hand, have generally been fitted into the vehicle dash ahead of the front seat passenger. Due to the relatively large space between the front seat passenger and the dash, these air bags have tended to be larger than driver side air bags. When fully inflated, a passenger side air bag will generally have a box-like configuration.
Due to various considerations, driver side air bags and passenger side air bags have often been constructed of different materials. Specifically, driver side air bags have frequently been constructed of a base fabric of either nylon or polyester which has been coated with chloroprene (neoprene), silicone or other appropriate elastomeric resin to reduce permeability. Passenger side air bags have generally been constructed of uncoated fabric.
It is important to design an air bag such that a specific rate of deflation is achieved. In other words, the air bag should quickly deflate in a controlled manner as it is impacted by the vehicle occupant. Adequate support will thereby be provided to the vehicle occupant without excessive rebounding.
To achieve the desired rate of the deflation, driver side air bags have generally been constructed having relatively large vent holes through which the inflation gas is expelled. It should be appreciated that an air bag intended to be used in a vehicle having a stiff frame will generally be required to deflate faster than an air bag for use in a soft frame vehicle. Thus, the specific size of these vent holes will generally be related to the crash pulse of the vehicle.
Because the inflation gas is generally very hot, the vent holes have typically been defined in the rear panel of the air bag opposite the front panel which is impacted by the driver. While face burns are largely avoided by placing the vent holes in this location, finger and hand burns have often occurred. Additionally, relatively large vent holes often allow sodium azide particulate in the inflation gas to escape into the vehicle's passenger compartment.
Due to these problems, the air bag industry has developed various alternative designs which do not have large vent holes. One such design is referred to as a "hybrid" air bag. The hybrid air bag is a driver side air bag utilizing a coated front panel which is generally impermeable to air, while having a back panel constructed of an uncoated fabric. This uncoated fabric, like the uncoated fabric utilized to produce many passenger side air bags, provides a degree of air permeability by venting air through the fabric's natural interstices.
Prior art uncoated fabrics utilized in vehicle air bags rely heavily on processing parameters to control air permeability. For example, different air permeability values have often been achieved by adjusting such factors as yarn preparation, weaving, scouring or heat setting. A problem with attempting to achieve a specific air permeability in this manner is that such processing parameters are often difficult to control on a consistent basis. As a result, relatively large variations in air permeability are often seen between respective lots of uncoated fabric which have ostensibly been prepared to the same permeability specification. In fact, variations of +/- three (3) cubic feet per minute (CFM) at 1/2 inch of water pressure are not uncommon. These wide variations in permeability may undesirably result in large variations in the deflation rates of respective air bags produced for a particular vehicle model.